The present invention relates to a method and device for removing coat spots from a printing plate of a printing machine wherein the printing plate is cleaned or wiped off by means of application roller of a three-roller moistening mechanism.
A moistening mechanism for supplying a moistening agent, e.g. moistening liquid, to a printing plate of the printing machine has been disclosed, for example in DE-OS No. 34 16 845. In this conventional moistening mechanism, the roller for the application of a moistening agent is driven by the drive of the printing machine due to the provision of a form-locking connection therebetween. The connection is carried out by a gear train drive which is provided with a push pressure-actuated coupling so that the moist application roller can be selectively utilized with or without the gear train drive. The drive ratios are selected so that in the coupled condition, the peripheral speed of the application roller is substantially lower than that of the printing plate, and the continual wiping effect (Delta-effect) takes place on the plate whereby the coat spots are removed from the printing plate.
A disadvantage of the convention device resides in that with a continual use of the Delta-effect the printing plate can wear off sooner. Periodic switchings-off of the Delta-effect within one printing application are not possible because very high differences in inking occur, which generate due to the difference in application of the moistening agent, a speed difference with or without Delta-effect.